Fix build error without precompiled headers

qstringlist.h no longer includes qiodevice.h

Change-Id: I08b83400316cb0b43bde0b390a9430e589e79e10
Reviewed-by: Robin Burchell <robin.burchell@viroteck.net>
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>

src/corelib/mimetypes/qmimedatabase_p.h

@@ -54,6 +54,7 @@
 
 QT_BEGIN_NAMESPACE
 
+class QIODevice;
 class QMimeDatabase;
 class QMimeProviderBase;